1. Field of the Invention
The present invention relates to an optical projection apparatus and method. In particular, this invention is related to image projection of opaque materials or transparencies onto a remote screen.
2. Description of the Related Art
In business or educational presentations, the commonly used overhead projectors are very useful tools. However, these projectors only accept transparencies. To project a handwritten or a printed document which are normally based on opaque materials, the document has to be converted into transparent slides through a special copying machine before projection. These converting steps are inconvenient. Moreover, converting printed materials with colors into transparencies is an expensive process. There is a definite need for optical projectors that project optical images from opaque materials. Projectors that project opaque materials are called episcopes, as differentiated from diascopes in which the objects to be projected need to be optically transparent for light rays to pass through. There are episcopes invented in the past but with certain limitations. Chief among all is that materials to be projected are mostly unaccessible to the operator. Examples of these types are U.S. Pat. Nos. 4,468,105 to Montgomery, Aug. 28, 1984; 4,343,538 to Astero Aug. 10, 1982; 3,920,323 to Nishitani, Nov. 18, 1975; 4,380,380 to Back, Apr. 19, 1983; and 3,619,048 to Engert et al., Nov. 9, 1971. With these episcopes, the object document to be projected must be fully enclosed. The reason is because there is a lack of an equivalent light concentration means such as a Fresnel lens used in the diascopic projectors to converge light rays from the object to the projection lens. Thus, light rays have to be confined within the episcopic enclosure from leaking out and a strong illumination within the enclosure is necessary to project a satisfactory screen image. This poses inconveniency for the operator to write on the document during presentations. There are episcopes with the document to be presented accessible to the operator. Examples are U.S. Pat. Nos. 4,565,430 to Grunwald, Jan. 21, 1986; and 3,512,883 to Noble, May 19, 1970. However, without a light concentration means in these episcopes as mentioned earlier, the illumination light source has to be extraordinary intense to achieve equivalent results. The strong light illumination on the object would cause discomfort to the operator's eyesight. Moreover, with the object to be projected under intense light and being exposed, unshielded light rays could transmit onto the remote screen and degrade the contrast of the projected image.
Part of the reasons for all the aforementioned inconveniences is because episcopes invented in the past almost all use lenses as light processing and magnifying elements. Ground lenses resulted from the manufacturing process are normally spherical in surface curvatures. Too large a lens aperture with a spherical surface curvature and a low f-number would incur all the optical defects such as spherical aberration and coma. It is for this reason that there is usually a limited aperture size associated with lenses which restrict the light utilization efficiency when lenses are used in episcopes. As a consequence, the majority of the light energy is not efficiently projected but instead is lost inside the enclosures of the episcope.
Another disadvantage related to prior arts that use lenses is another undesirable optical effect called chromatic aberration which occurs even for a lens with small aperture and a high f-number. Chromatic aberration is mostly eminent when a white light ray passes through a lens built of a homogenous material. The primary colors made up of the white light ray focus at different locations, because different colors assume different refractive indexes through the homogeneous lens material. The result is a sharp image for one color but blurred for the other colors. Combination lenses constructed with materials of different refractive indexes can be used to reduce the chromatic aberration effect, but that usually involves very expensive achromatic lens systems especially if a large aperture is required.
It is the object of the present invention to provide an optical projecting apparatus that projects not only transparencies but also opaque materials, without going through the inconvenient steps of transparent slide conversion, but still maintains all the conveniences of the conventional overhead projector.
It is another object of the present invention to provide sufficient brightness and contrast for the projected image on the remote screen without resorting to excessive intense light source illumination inside the apparatus due to the efficient usage of the light source of the present invention. As a consequence, the object under projection would be accessible to the operator but avoiding the shortfall of causing discomforts to the operator's eyesight from the intense light glares reflected by the exposed object. It also minimizes the spill-over of stray light rays reflected by the exposed object towards the remote screen and creates image contrast degradation.
It is a further object of the present invention to provide high quality images free of chromatic aberrations and other optical defects such as spherical aberration and coma without resorting to the inclusion of complicated optics.
It is still a further object of the present invention to furnish a projecting apparatus which is compact in physical size with the most efficient use of projection spaces due to the inherent foldable optical path of the present invention.